The eukaryotic translation initiation factor eIF4E (“4E”) is involved in the modulation of cellular growth. Moderate overexpression of 4E leads to dysregulated growth and malignant transformation. Both the nuclear and cytoplasmic function of 4E contribute to its ability to transform cells. Overexpression of 4E in vivo results in frank tumor formation, and the onset of tumor formation is greatly enhanced when 4E overexpression is placed within the context of a myc mouse background, suggesting again that 4E acts in concert with other oncogenes to promote neoplastic transformation. 4E is believed to represent one of the seven genes whose expression, when up-regulated in cancers, is predictive of metastatic disease. A variety of studies have been done demonstrating that existence of elevated 4E activity within surgical margins is a poor prognosis factor.
In the cytoplasm, 4E is required for cap-dependent translation, a process highly conserved from yeast to humans. 4E is believed to bind the methyl-7-guanosine cap moiety present on the 5′ end of mRNAs and subsequently recruits the given mRNA to the ribosome.
In the nucleus, 4E is a critical node in an RNA regulon that impacts nearly every stage of cell cycle progression. Specifically, 4E coordinately promotes the mRNA export, and in some cases also translation, of several genes involved in cell cycle progression. For example, 4E functions to promote export from the nucleus to the cytoplasm of at least two mRNAs, cyclin D1 and ornithine decarboxylase (ODC), while having no impact on the nuclear to cytoplasmic transport of GAPDH or actin mRNAs. Moreover, there is evidence that the mRNA export function of 4E is linked to its oncogenic transformation activity.
Dysregulated expression of tumor suppressors and oncogenes that maintain and enhance the malignant phenotype have been described. Among these molecules are tumor suppressors like p53, Rb, and APC and oncogenes such as myc, cyclin D1 and 4E. Their interaction constitute a network of self-reinforcing feedback loops wherein inactivation of principal elements can lead to the reversal and at times even the sustained loss of the neoplastic phenotype.
4E is overexpressed in a wide variety of malignant cell lines and primary human tumors including tumors of the breast, colon, head and neck, thyroid, lung, non-Hodgkin's lymphoma, prostate, cervix, bladder and chronic and acute myelogenous leukemias. Consistently, even moderate overexpression of 4E in rodent cells leads to deregulated proliferation and malignant transformation.
Despite being essential for growth and survival of eukaryotes by acting at a critical step of cap-dependent translation and recruiting transcripts to the ribosome as a result of its specific interaction with the 5′ 7-methylguanosine mRNA cap structure, up-regulation of 4E does not increase translation of all cap-dependent transcripts, but only of a specific subset of 4E-sensitive transcripts.
As much as 70% of 4E is present in the nuclei of mammalian cells, where it associates with nuclear bodies in a wide variety of organism, including yeast, Xenopus and humans. Here, 4E promotes transport of mRNAs of a specific subset of transcripts such as cyclin D1, but not of housekeeping genes such as B-actin and GAPDH. Post-transcriptional regulation of gene expression at the level of 4E mediated mRNA transport and translation exhibits different gene specificities, with some genes being regulated at the level of transport (e.g. cyclin D1) and some at the level of translation (VEGF), others at both levels (ODC), and still yet others at neither level (GAPDH). Binding to the m7G cap is required both for mRNA transport and translation by 4E, both of which contribute to this ability to transform cells.
Past observation indicates that 4E's capacity to discriminate between cyclin D1 and GAPDH is surprising seeing that the traditional view is that 4E binds the m7G cap found on all mRNAs regardless of other sequence specific features. Thus, this functional discrimination presents a conundrum in terms of our understanding of 4E mRNA recognition in the nucleus.
Elevated 4E activity has been observed to mediate selectively the translation (but not transcription) of a subset of the total collection of mRNAs expressed within cells, tissues, organs. Specifically, within cells, tumors and/or cancers where 4E activity is present at elevated levels, the translation of mRNA transcripts possessing complex 5′UTR regions is selectively upregulated. The repertoire of genes whose translation is thereby upregulated in circumstances where elevated 4E activity exists is a who's who of genes known to be involved in the regulation of the cell cycle, angiogenesis, proliferation and the like.
Existing cancer therapies are not effectively targeted/selective, thereby forcing patients to experience significant toxicity and side effects and/or they are not capable of addressing a wide range of cancers; neither are they capable of transforming cancer from a terminal disease process to one that can be managed long-term as are many others diseases (cardiovascular, diabetes to name a few). Existing gene therapeutics are conditional replicating lytic viruses, vectors/viruses containing RNAs encoding prodrug (aka suicide genes), anti-angiogenic agents, immune regulatory cytokines, tumor suppressors toxins and lytic peptides. Current gene therapeutic vectors/viruses are dependent upon the presence of elevated levels of 4E protein for the delivery of suicide genes, toxins, lytic peptides and/or proteins and/or processes.